I think they are referring to the header at the top which has a blue background.
Also the colored bars between the posts are blue, as are many icons, any type that's bold faced, etc.
I know that vbulletin sites can be and are different. It's probably a choice when vbulletin is first set up.
I'm OK with it, no desire to change on my part. Three blue cars out of 4 at the top of the page, do you want to re-color them too?